In conventional starters a freewheel device is disposed between the pinion and the drive bush. The main function of the freewheel is to prevent the pinion driving the electric motor of the starter at too high a speed, likely to damage it, when the internal combustion engine starts.
The freewheel device with rollers also damps sudden vibrations in the torque transmitted between the drive bush and the pinion.
This type of freewheel device using rollers offers very high performance, especially if the excessive speed of the pinion relative to the drive bush continues for a long period, which happens in particular if the driver does not de-energise the starter immediately after the internal combustion engine starts.
This phenomenon is occurring all the more frequently now that the passenger compartments of modern vehicles are increasingly better insulated from the acoustical point of view, this soundproofing blocking perception by the driver of the starting of the internal combustion engine, generally by listening for variations in the sound of the engine.
More recent electronically controlled starters include means for detecting starting of the internal combustion engine and for controlling the starter contactor in such a way as to return the overrunning clutch to its rest position by means of a lever on which the contactor acts.
Thus starters of the above kind use electronic control modules for their electric motor which automatically de-energise the electric motor of the starter and simultaneously return the drive bush to its rest position at the rear in which the pinion no longer meshes with the flywheel of the internal combustion engine.
The "freewheel" phase of operation is therefore of extremely short duration and is independent of the reaction time of the vehicle driver.
A prior art freewheel design, in particular of the type using rollers, is therefore overengineered for its purpose, in particular in relation to the short time for which it operates.
A freewheel of the above kind is therefore too bulky, too heavy and too costly for its function.
Completely eliminating the freewheel device has already been proposed, but eliminating the freewheel also eliminates the damping function, which is to the detriment of the mechanical durability of the starter and its operating noise level, which results in particular from the fact that the resisting torque imposed by the internal combustion engine varies greatly over one rotation of the crankshaft because of the cyclic forces compressing the gases present in the cylinders of the engine.
U.S. Pat. No. 3,666,958 to Ruhle et al. also proposes an inertial overrunning clutch design in which the coupling ring has a conical surround the section of which decreases in the direction towards the drive bush. During starting the enlarged conical drive bush acting on the elastic coupling ring engages with the conical surround. In the above device a compression spring bears against the pinion and against a stop ring fixed to the armature shaft.
In the above design the compression spring has a double role: to pre-stress the two cones and to return the overrunning clutch to its rest position and retain it there. Accordingly, in the operative position, the spring develops a maximal force that increases the residual torque during freewheel operation although a low force is required to avoid transmitting excessive overspeed to the armature.